Devices of this kind are used primarily in information-transmission systems in which the information to be transmitted is coded in digital form and/or sampled, and is used to modulate a carrier by frequency shift or phase reversal for example. At the transmitter, the timed distribution of the successive symbols representing the information is effected on the basis of sync pulses furnished by a clock. At the receiver, in order to carry out proper demodulation, it is necessary for the demodulator to be synchronized with the modulator in the transmitter so as to establish when one symbol or bit ceases and the next begins. Synchronizing means are therefore provided in the system.
The synchronizing signal, in the absence of distortions occurring during transmission, will consist of a series of pulses of constant duration and a constant and accurately defined recurrence period. However, distortions during transmission mean that certain pulses disappear while others are time-shifted and mixed with parasitic pulses.